1. Field of the Invention
The present invention relates to a method and apparatus for observing, analyzing and inspecting crystal defects preferably applicable to analysis and inspection of internal crystal defects or junction fracture due to electrical stress, etc., of a semiconductor device, a compound semiconductor laser diode device in particular.
2. Related Background Art
As shown in FIG. 4, a general compound semiconductor laser diode device is constructed of a laser diode chip 1, a submount 2 on which the laser diode chip 1 is mounted, a stem 3 integral therewith, a photodiode 4 which detects optical output, a sealing glass-windowed cap 5, an anode 19a which applies a voltage to the laser diode chip 1 and a cathode 19b, etc. The laser diode chip 1 generally has a laminated structure of several types of semiconductor. As shown in FIG. 5, the laser diode chip 1 incorporates an oscillator (stripe) 8 for trapping laser light and amplifying laser light by resonance and the oscillator 8 is constructed of part of the laminated semiconductor. Furthermore, one end of the anode 19a, cathode 19b is connected to the laser diode chip 1 to apply a voltage to the laser diode chip 1. Many technologies have been conventionally developed for fault analysis and inspection of this compound semiconductor laser diode (hereinafter referred to as “laser diode”).
First, a near field pattern (NFP) observation method is generally well known which two-dimensionally captures a light-emitting state of the end face of the laser diode chip 1 at the laser light emitting position using an infrared camera, etc., observes the light-emitting form and compares it with a conforming product to decide abnormalities (“Insight Into Semiconductor Laser” written and compiled by Minoru Konuma, Mitsuyoshi Shibata, 2nd edition, Engineering).
However, since the NFP observation method observes only the end face which reflects laser beam, it can only detect traces of damage by COD (optical damage) fracture appearing as optical information on the end face. Therefore, it is difficult to check abnormalities such as crystal defects inside the chip.
Furthermore, an analysis, inspection method for observing the light-emitting state of the oscillator 8 called a “stripe observation method” is also known. The method of inspecting using this technique will be explained using FIG. 5. First, in order to make the whole oscillator 8 in the longitudinal direction observable from the top surface of the laser diode chip 1, the top electrode 6 which intercepts light transmission is chemically or mechanically removed. FIG. 5 shows the top electrode 6 whose part has been removed. When the entire surface is removed, a new electrode is formed at a position which will not obstruct observation of the oscillator 8 later. Then, in this condition, a voltage is applied from a voltage supply 122 between the anode 9a and cathode 9b of the laser diode chip 1. The upper electrode 9a and a lower electrode 7 beneath the laser diode chip 1 are connected to the anode 9a and cathode 9b respectively, and therefore a current is generated in the laser diode chip 1 and the oscillator 8 inside starts to emit light. This oscillator 8 which emits striped light is observed using an infrared camera 101, etc., and this state is compared with a conforming product to observe abnormalities.
The stripe observation method further includes a CL observation method which chemically or mechanically removes part from the oscillator 8 inside the chip to the outer surface of the chip until the thickness is reduced to an extent that electron beams pass and observes crystal defects, etc., through cathode luminescence (CL) of the oscillator 8. This principle will be explained. When electron beams are directly irradiated onto the oscillator 8 of the laser diode chip 1, electrons are inelastic-scattered by a sample and lose energy. Part of this energy excites a valence band and produces pairs of electron and hole. Electrons and holes scatter in the sample, recombine at certain positions and light is emitted at this time. This light emission reflects a band structure in the defect area, thereby produces a difference in light emission intensity and spectral shape from other normal areas and finding out this difference makes it possible to identify the defect.
According to the stripe observation method, when there are abnormalities such as crystal defects and fractures in an oscillator for checking the light emitting image, these areas can be observed as non-light-emitting parts in many cases, making it possible to identify the abnormal areas. However, since it is light emission without directionality, the boundary between the non-light-emitting areas and light-emitting areas, is ambiguous and in the case of micro abnormal areas in particular, these areas may not be detected due to blurring, etc., of light from the peripheral area. Furthermore, infrared light emitted from the oscillator is observed after passing through other parts of the chip, but visible light can hardly be transmitted, and therefore it is not possible to obtain physical information of the oscillator layer from the outside. For this reason, even if there are non-light-emitting areas (abnormal areas), it is only possible to identify those areas, whereas it is not possible to decide what physical condition those parts are in as the laser diode chip.
Since electron beams are used in the CL observation method, observation needs to be performed in vacuum, which leads to a large equipment cost. Furthermore, while the detection accuracy of crystal defects is excellent, it is necessary to reduce the thickness up to the oscillator to a thickness of approximately several microns so that electron beams can be irradiated onto the oscillator, which requires high precision machining and time during preprocessing of a sample. Moreover, as in the case of visible light, it is only possible to obtain information on the form and condition of the surface (approximately 1 micron in depth) of the device as in the case of visible light and it is not possible to obtain information on the chip interior.
(Reference: “Insight Into Semiconductor Laser” written and compiled by Minoru Konuma, Mitsuyoshi Shibata, 2nd edition, Kogakutosho Ltd. May 25, 1998, p111)
As the methods applicable to fault analysis and inspection of a semiconductor integrated circuit chip, OBIC (Optical Beam Induced Current) method and OBIRCH (Optical Beam Induced Resistance Change) method are known.
The OBIC method is an analysis and inspection method using an optically excited current generated by creation of pairs of electron and hole caused by transition between a valence band and conductive band due to light irradiation onto a Si semiconductor device and uses light having a wavelength with larger energy than the energy gap of the target device. For example, in the case of Si semiconductor, using He—Ne laser light having a wavelength of 633 nm, an OBIC current which is an optically excited current is generated efficiently to detect defective areas.
The OBIRCH method scans and irradiates internal mutual wiring in a semiconductor integrated circuit with laser light as visible light and heats it, detects a change in resistance caused by a temperature rise through irradiation and a change of the current flowing through the wiring and detects defects in the wiring (U.S. Pat. Nos. 5,422,498 and 5,804,980).
However, the OBIC method is an inspection method for observing an optically excited current and irradiates light having a wavelength with greater energy than the band gap energy of Si semiconductor. The laser beam used is, for example, 633 nm He—Ne laser, etc., and the problem is that there is a limit to the wavelength of light that can be used.
The OBIRCH method cannot detect crystal defects in the area without wiring such as inside the laser diode chip.